1. Field of the Invention
The present invention is directed to dual mechanical seals, such as a face seals, of the type wherein an inboard seal is exposed, in a stuffing box, to a process fluid and an outboard seal is exposed to a pressurized barrier fluid.
2. Description of the Prior Art
In mechanical seals known as dual face seals, two pairs of relatively rotating seal faces, such as two seal faces affixed in some manner to a stuffing box housing and two rotating seal faces fixed to the shaft of rotary equipment, are employed to isolate and seal a pressurized liquid, referred to as the process fluid, along the rotating shaft. In order to cool the seal faces and to assist in preventing passage of process fluid across the seal faces, a second pressurized liquid, a barrier fluid, is often introduced between the two seals.
Seals of this general type are commercially available, for example, from A. W. Chesterton Company under product model nos. 225, 255 and 280. In such seals, the seal which is exposed to the process fluid is referred to as the inboard seal, and the seal which is (ideally) only exposed to the barrier fluid is referred to as the outboard seal.
The barrier fluid in such seals is usually selected dependent on properties such as lubricating capability, heat conveying or heat dissipation capability and compatibility with the process fluid being sealed, the seal components and to the environment, since a small amount of the barrier leak into the process and/or the environment. The process fluid, by contrast, may be any type of industrial fluid which must be conveyed from one location to another by means of a pressure differential in a line containing the process fluid typically by means of a pump containing mechanical seals. In most instances it is desired to keep the barrier fluid and the process fluid separated as completely as possible, to avoid either contaminating the process fluid with barrier fluid, or vice versa. In cases such as extremely corrosive, toxic, radioactive, or volatile organic compound process fluids, it is desirable to avoid contaminating the barrier fluid with the process fluid because this will then result in a mixture of the process fluid and barrier fluid leaking across the outboard seal and into the environment. In some instances, the necessity of avoiding this leakage into the environment has required the use of sealess pumps, special gas seals or a piping system to a plant incineration system to burn off undesired gases, or a vapor recovery system to prevent gases from escaping into the atmosphere. Additionally, if the process fluid is intended for human use or consumption, such as a component of a food product or a cosmetic product, there is often a necessity to prevent the process fluid from being contaminated by the barrier fluid.
In known systems making use of a barrier fluid, contamination of the process fluid by the barrier fluid, and vice versa, has been attempted to be accomplished primarily by confinement, i.e., by trying to maintain the mechanical locations at which the barrier fluid and process fluid have an opportunity to mix as few as possible and as small as possible. In a barrier seal system disclosed in U.S. Pat. No. 5,249,812, for example, the barrier fluid is pressurized from a gravity feed line, and a barrier fluid containment device is attached to the high pressure side of the seal in an effort to prevent process fluid from contaminating the seal faces. In a stuffing box lubricator disclosed in U.S. Pat. No. 5,305,854, a check valve in the fluid flow path between the stuffing box and the lubrication reservoir prevents the flow of spent hydraulic fluid from the stuffing box into the lubrication reservoir in the event of a seal leak or failure.
In a hydrostatic face seal and bearing assembly disclosed in U.S. Pat. No. 5,052,694, a high pressure cavity is provided in one of two opposed sealing faces between a shaft and a housing for supplying a pressurized barrier fluid between the opposed sealing faces. Passage of the barrier fluid into the interior of the housing and passage of the process fluid from the housing into the high pressure cavity is stated to be achieved by employing a circumferential dam which forms a narrow gap between the high pressure cavity and the interior of the housing, and a system is also provided for maintaining the pressure of the barrier fluid in the high pressure cavity substantially equal to the pressure of the process fluid in the housing. A sealing mechanism for a reciprocating apparatus is disclosed in U.S. Pat. No. 4,769,992 wherein a fluid such as oil is enclosed in a fluid chamber formed by a pair of flexible partitioning members, such as diaphragms or multiple bellows, and the sidewalls of a cylinder. The fluid chamber is divided into two parts by a pressure-resistive sealing member which is affixed to an inside wall of a rigid partition, so as to roughly seal the fluid while still permitting sliding motion of the reciprocating member within the cylinder.
A shaft seal is disclosed in U.S. Pat. No. 4,128,248 in the form of a dynamic packing having a wall which separates an oil-containing space from an oil-free space, or from the atmosphere, formed by two shaft sealing rings which engage the shaft and which define an oil-filled gap therebetween. Respective conduits lead from the oil-free space and from the oil-filled gap and terminate at one side of a diaphragm, the opposite side of the diaphragm being exposed to a reference pressure, for the purpose of relieving excess pressure in the oil-filled gap.
An apparatus for preventing leakage from an enclosed space or a circulating system which contains a noxious fluid is disclosed in U.S. Pat. No. 3,161,413; wherein a compensating chamber is in fluid communication with each of a "clean" chamber and a "contaminated" chamber, the compensating chamber having an element therein, such as a piston or diaphragm, which physically separates the contaminated chamber from the clean chamber, but which permits a steady state pressure equilibrium to be reached between the other two chambers.
Another type of packing arrangement for a stuffing box is disclosed in U.S. Pat. No. 943,169; wherein a pressurized fluid is maintained at a selected pressure by a screw operated piston contained in a fluid reservoir.
Lastly, a shaft seal is disclosed in U.S. Pat. No. 2,127,591 having a bellows arrangement which is in communication with a chamber disposed between two sealing surfaces, the bellows maintaining the chamber at a pressure which slightly exceeds a pressure in the surrounding crank case so that if any leakage occurs, it is from the interior of the chamber into the crank case, rather than vice versa. It is also stated in U.S. Pat. No. 2,127,591; however, that the pressure in the interior of the chamber and the pressure in the crank case can be substantially balanced.